water mixture by organophilic nano-silica filled PDMS composite membranes

water mixture by organophilic nano-silica filled PDMS composite membranes

Desalination 322 (2013) 159–166 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Per...

1MB Sizes 0 Downloads 126 Views

Desalination 322 (2013) 159–166

Contents lists available at SciVerse ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Pervaporation of ethanol/water mixture by organophilic nano-silica filled PDMS composite membranes De Sun a,b, Bing-Bing Li b, Zhen-Liang Xu a,⁎ a State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China b Department of Chemical Engineering, Changchun University of Technology, 2055 Yanan Street, Changchun 130012, China

H I G H L I G H T S • Organophilic nano-silica filled polydimethylsiloxane membranes are prepared. • The operation conditions of pervaporation for the ethanol–water mixture were studied. • These composite membranes exhibit striking performances.

a r t i c l e

i n f o

Article history: Received 6 January 2013 Received in revised form 4 April 2013 Accepted 8 May 2013 Available online 13 June 2013 Keywords: Organophilic nano-silica PDMS composite membranes Pervaporation Ethanol–water

a b s t r a c t A novel organophilic nano-silica (ONS) filled polydimethylsiloxane (PDMS) composite membrane was prepared and characterized for the pervaporation (PV) of ethanol from water. The sorption and diffusion behaviors of ethanol and water in the composite membranes were investigated. The results showed that with increasing ONS concentration from 0 wt.% to 10 wt.%, the solubility selectivity and the diffusion selectivity increased from 21.87 to 38.85 and from 1.87 to 6.18, respectively. When the temperature ranged from 30 °C to 70 °C, the solubility selectivity increased but the diffusion selectivity first increased and then decreased. The effects of ONS content, feed temperature and permeate-side vacuum on the PV performance of the composite membrane for the pervaporation of 5 wt.% ethanol–water mixture were studied. The examinations showed that the composite membranes exhibited striking advantages in total flux and separation factor as compared with unfilled PDMS membrane. When ONS loading was 5 wt.%, the PDMS composite membrane showed the best PV performances with the permeate flux (J) of 114 g/(m2 · h), the separation factor (α) of 30.1 and permeate separate index (PSI), of 3420, respectively. With an increase of the feed temperature from 30 to 70 °C, the total flux of filled PDMS membrane with 5 wt.% ONS increased apparently from 11.1 to 210 g/(m2 · h), and the maximum separation factor of 30.1 was observed at 60 °C. With an increase of vacuum in permeate-side from 0.075 to 0.100 MPa, both separation factor and total flux increased for filled PDMS membrane with 5 wt.% ONS. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Membrane separation technique is one of the most promising achievements of energy-saving technologies. Organophilic separation of alcohol/water mixture by using the PV technique has been studied widely for its potential applications in the areas of biochemical engineering, food and beverage industry and environmental engineering [1–7]. The process is expected to be integrated into an alcohol fermentation scheme in which continuous attract ethanol is obtained during the fermentation so as to achieve continuous alcohol fermentation which in turn improving the production of ethanol from fermentation operation. Investigations [8–10] showed that it is very important to ⁎ Corresponding author. Tel./fax: +86 21 64252989. E-mail address: [email protected] (Z.-L. Xu). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.05.008

enhance the solubility of alcohol over water in a polymer membrane due to the higher diffusivity of water compared with alcohol. Among the most common PV polymers, such as poly(dimethylsiloxane) (PDMS) [10,11], polyether block polyamide (PEBA) [12], crosslinked poly (acrylate-co-acrylicacid) [13,14] and polyvinylidene fluoride (PVDF) [15], PDMS is good for its flexibility because of its amorphous molecular structure and rubbery state at ambient temperature (Tg = −123 °C). It has been found PDMS has a higher selectivity for ethanol [16]. To improve the PV performance and physicochemical properties of polymer membranes, several kinds of inorganic fillers such as polyphosphazene nanotube, nano-silica, carbon nanotube, zeolite, nano-iron, TiO2 and carbon black [10,17–25] have been introduced into the PDMS polymer casting solutions. It has been reported that nano-silica filled polymer matrix composites showed a significant improvement in physicochemical stability and separation performance

160

D. Sun et al. / Desalination 322 (2013) 159–166

[18,20,26]. However, it seems to be difficult to obtain well-dispersed inorganic–organic nanocomposites prepared by blending method due to the strong tendency of aggregation in inorganic nanoparticles which may cause the depressive properties of nanocomposites materials. Consequently, it is expected that a well-dispersed inorganic–organic nanocomposites possessed improvement in permeate performance and physicochemical properties can be prepared and be applied to concentrate organic components from dilute aqueous solution by PV. Zhou et al. [19] treated the nano-sized silicalite-1 with a silane coupling agent vinyltriethoxysilane (VTES) to prepare the modified silicalite-1 and incorporated it into PDMS matrix for the PV of dilute ethanol solutions. The results showed that VTES could enhance the interaction of silicalite-1 particles with PDMS through chemical bonds and the VTES modified silicalite-1/PDMS hybrid membranes could effectively improve the PV selectivity at different silicalite-1 loadings. In this case, the industrial organophilic nano-silica (ONS) filled PDMS composite membranes supported by non-woven fabrics were prepared with different content of ONS additive by solution casting method. The physicochemical properties of the filled PDMS composite membranes were systematically investigated using SEM, FT-IR and XRD. The sorption and diffusion behaviors of ethanol and water in the composites were also investigated. The effective conditions, such as ONS content, feed temperature and downstream vacuum, were discussed for the PV of ethanol aqueous of 5 wt.% which is the typical concentration of an alcoholic fermentation broth. 2. Experiment 2.1. Materials Non-woven fabric as the membrane support layer material was obtained from Changzhou Haoxin Insulation Material Co. Ltd. (China). Polydimethylsiloxane (PDMS) (Silicone Rubber 107, Mw5000), cross linking agent ethyl silicate and curing agent dibutyltin dilaurate were purchased from Shanghai resin Company (China). Organophilic nanosilica (H15, diameter = 50 nm, BET surface area = 120 m2/g, Average pore size = 15 nm) used as additive supplied by Guangzhou Hualisen Chemical Material Co. Ltd. (China). Reagent grade n-heptane was obtained from Shanghai Ruen Jie chemical reagent Company (China). Commercially supplied ethanol was used for PV experiments without further purification. 2.2. Membrane preparation The PDMS membranes were prepared by solution casting method and the casting solution was prepared by dissolving PDMS, crosslinker (ethyl silicate) and curing agent (dibutyltin dilaurate) in the solvent (n-heptane) with a ratio of 10:1:0.5 (in weight). The solution was subjected to homogenize using a magnetic stirrer for about 3 h. To prepare the cross linked unfilled PDMS flat sheet composite membrane, the homogeneous casting solution was poured onto the surface of a non-woven fabric for 15 s and then to be dried in the sterile room at room temperature for 24 h. The total membrane thickness was about 130.0 ± 10 μm determined by micrometer. For the preparation of the ONS filled PDMS membranes, ONS nanoparticles were dried for 24 h at 70 °C in a vacuum oven and then were evenly dispersed in a newly prepared PDMS casting solution under stirring for 3 h. The rest of the preparation process was just as the same as that of unfilled membranes. The total membrane thickness was about 130.0 ± 10 μm determined by micrometer. To investigate the effect of ONS on physical characterization and PV performance of the PDMS membranes, the ONS filled PDMS composite membranes with different contents of ONS were prepared. For simplicity, membrane samples were designated as 0% ONS-PDMS, 1.5% ONS-PDMS, 5.0% ONS-PDMS, 10% ONS-PDMS, 15% ONS-PDMS, 20% ONS-PDMS and 25% ONS-PDMS according to the ONS content.

In addition, to study the effects of the physicochemical properties on the filled membranes, unfilled and filled PDMS membranes without support were also prepared. 2.3. Membrane characterization The membrane samples were fractured in liquid nitrogen and then coated with gold to observe the top surface, cross-section and bottom surface structures by using a scanning electron microscope (SEM) (JEOL Model JSM-5600 LV, Japan). Fourier-transform infrared spectroscopy (FT-IR) spectra of the samples were recorded in the 500–4000 cm−1 range using a Nicolet-560 spectrometer (Nicolet, America). X-ray diffraction spectra of the ONS and the ONS filled PDMS membranes were obtained at room temperature using a D-MAXIIA X-ray diffractometer (RIGAKU, Japan). The diffractograms were measured at a scanning speed of 10°/min in the 2θ range of 5–60° by means of a tube voltage of 40 kV and tube current of 30 mA. The contact angle of water was measured by a JC2000D1 contact angle meter (CA-D type, Shanghai Zhongcheng Digital Technology Apparatus Co. Ltd., china) at RT and 60% relative humidity. Water droplets (sessile drops volume ca. 0.2μL) were placed on the membrane and after 10 s the dimensions of the droplets were measured using the system software. 2.4. Swelling experiments Membrane swelling experiments can help to understand the interactions between the membranes and the liquid penetrants. To make the swelling membranes, pieces of dried PDMS composite membrane samples without support samples were weighed using a highly sensitive electronic balance (ALC-1100.2, sartorius, German) with an accuracy of 0.0001 g and then were immersed in either ethanol aqueous solution, pure ethanol or water at different temperatures. After each time interval, the membrane was took out and wiped off the surface liquid and were weighed immediately. The equilibrium solvent uptake was measured until no significant weight increase was observed for the swollen membrane. The data in this paper are average values of four to five measurements. The degree of swelling of the membrane in ethanol aqueous solution, W∞, was determined by W ∞ ¼(M ∞ −M 0 )=M0

ð1Þ

where M∞ is the mass of the fully swollen membrane and M0 is the mass of the dried membrane. The sorption ideal selectivity [27], αs, was calculated by α s ¼ W E;∞ =W W;∞;

ð2Þ

where WE,∞ and WW,∞ are the swelling degrees of the membranes in the pure ethanol and water, respectively. The diffusion coefficient (D) of the water and ethanol in the membranes were measured from the initial linear sorption curves [27]. h  i0:5 2 Mt =M ∞ ¼ 4 Dt πL

=

ð3Þ

where Mt refers to the solvent mass sorbed at time t. L is initial thickness of membrane. The diffusion coefficients of the water and ethanol were obtained from the slopes of plots of Mt/M∞ versus t1/2 according to above equation. The diffusion selectivity (αD)was calculated by α D ¼ DE =DW

ð4Þ

where DE and DW are the pure ethanol and water diffusion coefficients, respectively.

D. Sun et al. / Desalination 322 (2013) 159–166

2.5. PV process for the separation of ethanol/water mixtures Fig. 1 shows the schematic diagram of PV apparatus used in this case. PV experiments were conducted using a cross-flow laboratory scale membrane unit with a relatively small effective membrane area of 5.5 × 10−3 m2. Coupled with a large feed tank (V = 3 L), it allowed the determination of the PV fluxes at almost constant feed concentration. Dilute (5 wt.%) ethanol/water solution was used as feed keeping in the feed tank circulated by a circulation pump. The feed solution was pumped into the membrane cell with the high flow rate of 50 L/h to minimize the effect of concentration polarization. The feed tank was kept in a water bath at a definite temperature controlled by a temperature controller. The permeate-side vacuum pressure was controlled using a vacuum pump. After the operation reached a steady state (about 1 h after started), the permeate vapor samples was collected in a cold trap using liquid nitrogen, then the sample was weighed and analyzed by gas chromatography with a thermal conductivity detector(TCD) (Techcomp LTD, GC7890). The calculation of the permeation flux, separation factor and permeate separate index are defined as J ¼ m=(△t  A)

ð5Þ

α ¼(yalcohol =ywater )=(xalcohol =xwater )

ð6Þ

PSI ¼ ðα−1ÞJ

ð7Þ

where m is the total amount of permeate collected during the experimental time interval △ t of 1 h at steady state, A is the effective membrane area, x and y represent the mole fraction of a component in the permeate and the feed, respectively.

161

matrix due to the good compatibility between organophilic ONS particles and organophilic PDMS. Therefore, no nonselective defects voids could be found at the interface of the PDMS and nanosilica particles as shown in the picture. Thus it can be expected that the selectivity of ethanol to water in PV conditions could be increased by the higher sorption selectivity of ONS filled membrane [17]. The cross-section image (as shown in Fig. 2(b)) of the 5.0% ONS-PDMS composite membrane showed that the active layer of about 80 μm in thickness was tightly adhered on the surface of the non-woven fabric support layer. The bottom surface SEM image showed that there was no PDMS particle permeate into the support layer as shown in Fig. 2(c). 3.1.2. FT-IR analysis Fig. 3 shows the FT-IR spectra of ONS (a) and ONS filled PDMS membranes with various ONS content (b). As exhibited in Fig. 3(a), the absorption peaks at 3440 cm−1 and 1110 cm−1 correspond to stretching and asymmetric stretching vibrations of Si–OH on the ONS surface, respectively. The peaks of 803 cm−1 and 467 cm−1 represent the asymmetric stretching and bending vibration of siloxane groups (Si–O–Si), respectively. A weak band at 2960 cm−1 is ascribed to symmetric vibration of the hydrophobic C–H groups in the FT-IR spectrum of ONS [28]. We can see from Fig. 3(b), all membranes had very similar spectra peaks at a wave range from 500 cm−1 to 4000 cm−1. The absorption peaks at around 1072 and 1009 cm−1 in the filled membranes are corresponded to stretching vibrations of Si–O–Si. The peaks at 1255 cm−1 and 1415 cm−1 are assigned to deformation vibration and dissymmetry deformation vibration of the two methyls linked with Si. The characteristic peaks at around 786–872 cm−1 and 2863–2966 cm−1 represent the stretching vibrations of Si–C and C–H, respectively. Compare the spectrums of unfilled membrane with that of the filled PDMS membranes, no new absorption peak could be observed, which demonstrates that the ONS is only physically blended with the polymer matrix [21].

3. Results and discussion 3.1. Membrane characterization 3.1.1. SEM analysis In order to investigate the distribution of ONS in the filled membranes, SEM characterizations of the top surfaces of the ONS-PDMS composite membranes, which have different ONS content have been carried out. As shown in Fig. 2(a), the filled PDMS membrane is dense with no appreciable voids and ONS dispersed uniformly in the PDMS

3.1.3. XRD analysis Fig. 4 shows the XRD spectra of ONS and ONS filled PDMS membranes with various ONS content. As illustrated in Fig. 4, the ONS exhibits a typical amorphous peak at about 21.59° and the unfilled PDMS membrane exhibits typical amorphous peaks at about 11.91° and 21.47° [21]. For the filled PDMS membranes, the increase of ONS content leads to lower increase in the peak intensities at about 21.59° in comparison with the unfilled PDMS membrane, which implies that the incorporation of ONS into PDMS membrane has a weak effect on the crystalline characterization of filled PDMS membrane. 3.1.4. Contact angle analysis Table 1 shows the experimental values of the water contact angles on the air-side surface of the ONS filled PDMS membranes with different ONS content. It can be seen that the water contact angles increased with the increase of ONS content in PDMS, which suggests that the more ONS content in filled PDMS membrane, the higher hydrophobicity of the filled membrane surface. When the ONS content increases from 0% to 25%, the contact angle of the filled membrane surface increased from 109.0° to 125.0°.

Fig. 1. Schematic diagram of pervaporation apparatus.

3.1.5. Membrane swelling analysis Degree of swelling is important to understand PV performance [29]. Degree of swelling of PDMS composite membranes with different ONS contents soaked in 5 wt.% ethanol/water mixtures at 60 °C for 2 days were presented in Fig. 5. We can see that the incorporation of ONS into PDMS membrane enhances the swelling degree and the swelling degree of these composite membranes increase with the increase of ONS content. This may be due to the fact that the physical incorporation of ONS particles as analysized in the Sections 3.1.2 and 3.1.3 can interfere the tight packing of PDMS chains [20], which enhances the accessible free volume in the matrix, which in turn facilitates the sorption of water and ethanol molecules [1]. A similar effect

162

D. Sun et al. / Desalination 322 (2013) 159–166

Fig. 2. SEM photographs of (a) top surface (5,000×) of the ONS-PDMS composite membranes with different ONS contents, (b) cross-section (500×) and (c) bottom surface (200×) of the 5.0% ONS-PDMS composite membrane.

was observed by Qing-Lin Liu et al. [30] who used Silicalite-filled poly (siloxane imide) (PSI) membranes to study the sorption behavior in aqueous solution of 1.2 wt.% chloroform at 50 °C.

Fig. 3. FTIR spectra of ONS (a) and ONS filled PDMS membranes with various ONS content (b).

Fig. 6 shows the typical sorption history of the pure ethanol (a) and water (b) in PDMS composite membranes at 60 °C. The PDMS composite membranes have higher affinity to ethanol than to water due to the bigger chemical compatibility of ethanol with the polymer as analyzed in Section 3.1.4. All the PDMS composite membranes showed higher ethanol sorption than water does. When 5% ONS was incorporated into the composite membrane, the ethanol uptake in the composite was 0.0634 kg/kg, and the water uptake was 0.0023 kg/kg. The effect of the ONS addition on the solubility selectivity and diffusion selectivity for the pure ethanol over water at 60 °C is shown in Fig. 7(a) and (b) which can help to understand the following pervaporation performances. With an increase of ONS addition, the ethanol uptake increased due to the promoted hydrophobicity caused by the incorporation of ONS into PDMS (as shown in Table 1), but the water uptake firstly increased and then decreased. For the increase of water uptake, the reason may be due to a capillary condensation mechanism interpreted by H. Ahn [31]. Both ethanol and water

Fig. 4. XRD spectra of ONS and ONS filled PDMS membranes with various ONS content.

D. Sun et al. / Desalination 322 (2013) 159–166

163

Table 1 Water contact angle for ONS filled PDMS membranes. ONS content/wt.%

0

1.5

5

10

15

20

25

The contact angle/°

109.0

112.5

116.5

117.5

119.0

121.5

125.0

diffusion coefficients firstly increased then decreased with the increase of ONS content in PDMS. The reason is that when the ONS content was below 5 wt.%, the loose packing structure of PDMS chains caused by the incorporation of ONS is main control factor for diffusion, but when the ONS content was above 5 wt.%, the main control factor might be the long and tortuous diffusion path caused by the big surface area of BET and mesoporous pore structure of ONS. When the ONS loading was increased from 0 to 10% in the PDMS, the solubility selectivity and the diffusion selectivity have the same trends which increased from 21.87 to 38.85 and from 1.87 to 6.18, respectively. These data suggest that the ONS filled PDMS membranes preferentially favored to the sorption and diffusion of ethanol, which is desirable for concentrating ethanol from aqueous solutions. The effect of feed temperature on the sorption and diffusion behaviors of ethanol and water in 5.0% ONS-PDMS composite membrane is shown in Fig. 8(a) and (b), respectively. With an increase of operating temperature from 30 to 70 °C, both the ethanol and the water uptakes increased, which can be interpreted by the increase of the mobility of polymer chains and the increase of free volumes of the ONS filled matrix. In addition, both ONS and PDMS used in this study are hydrophobic, which results in a steeper rise of the uptake towards ethanol than the uptake towards water, so we can see solubility coefficient increased with the increase of feed temperature. The diffusion coefficients increased due to the enhanced free volumes of the ONS filled matrix caused by the increase of temperature as showed in Fig. 8 (b). The ethanol diffusion coefficiency increases at a higher rate than that of water when temperature was below 60 °C, but when temperature is 70 °C, the ethanol diffusion coefficient increases at a lower rate than that of water. So we can see that the diffusion selectivity firstly increased then decreased, which is important to define the feed temperature for ethanol/water mixtures pervaporation.

Fig. 6. Effects of ONS content on the solvent sorption history for pure ethanol (a) and water (b) in PDMS.

compared with unfilled PDMS membrane. A similar effect was observed by Adnadjevidć etal. [32] that the addition of the three investigated types of hydrophobic zeolites results in the improvements of

3.2. PV performance 3.2.1. Effect of ONS content in the membrane on PV performance The effects of ONS content on total flux, separation factor and permeate separate index of PDMS composite membranes are shown in Fig. 9 (a) and (b) at the 5 wt.% ethanol concentration, feed temperature 60 °C and permeate-side vacuum 0.100 MPa. As is shown, the moderate filling of ONS exhibits striking advantages in the flux and the separation factor for PV separation of ethanol/water mixture

Fig. 5. Effect of the ONS content on the degree of swelling of PDMS composite membranes.

Fig. 7. Effects of ONS content in membrane on the solubility selectivity (a) and diffusion selectivity (b) for the pure ethanol over water.

164

D. Sun et al. / Desalination 322 (2013) 159–166

Fig. 8. Effects of operating temperature on the solubility selectivity (a) and diffusion selectivity (b) for the pure ethanol over water. Fig. 9. Effects of ONS content in membrane on the total flux, separation facter (a) and permeate separate index (b) of ONS filled PDMS composite membranes.

membrane selectivity and the total flux for PV of ethanol aqueous solution. As can be seen from Fig. 9(a), with the increase of ONS content from 0 wt.% to 25 wt.%, the total flux increased quickly to the maximum 114 g/(m2 · h) when ONS content was 5 wt.% then decreased slowly, similar changing trend was also found for polyphosphazene nanotubes (PZSNTs) containing membrane[17]. The separation factor increased from 14.1 to 41.2, a 3 times improvement. This correspond well with data obtained by Zhou et al.[19] who used the modified silicalite-1/PDMS membrane in separation of acetone, butanol, ethanol (ABE) model solution. When ONS was added into PDMS membrane, as analyzed in Section 3.1.3, it can interfere with the tight packing of PDMS chains [8], which makes the diffusion of the permeating molecules through the filled membranes easier (as shown in Fig. 7(b)). Additionally, due to organophilic properties of the ONS, more ethanol molecules can be preferentially sorbed and diffused in the membranes (as shown in Fig. 7). Therefore, compare the 5 wt.% ONS filled membranes with the unfilled membrane, we can see a rapid enhancement of the total flux and the separation factor. But with the further increase of ONS content in the PDMS membrane, for the following two reasons the total flux decreased. Firstly, with increase of ONS, the hydrophobicity of the surface of filled membrane increased. Secondly, the big BET surface area and mesoporous pore structure of ONS make the water permeate path in the filled membranes long and tortuous [18], which caused the decrease of water sorption and diffusion(as shown in Fig. 7). Thus the total flux firstly increased and then decreased but the separation factor always increased with the increasing of ONS filler. In Fig. 9 (a), the changing trends of the separation factor and the total flux are contradictive when ONS content is bigger than 5 wt.%, so we introduce PSI to evaluate the permeation performance of the ONS filled membrane. It is shown in Fig. 9 (b) that the PSI has the similar trend with the total flux and the 5 wt.% ONS filled membrane has the best PV performance in all tested ONS filled PDMS membrane.

3.2.2. Effect of feed temperature on PV performance Fig. 10 (a–c) shows the effect of feed temperature on PV performance of the 5.0% ONS-PDMS composite membrane at the ethanol concentration of 5 wt.%, permeate-side vacuum 0.100 MPa. Generally, with the increase of operating temperature, total flux increases and separation factor decreases [10,25]. But in this study, it appears that as the feed temperature increases, the total flux increased significantly and the separation factor increased slightly to a maximum then decreased and the maximum value 30.1 is observed at 60 °C as illustrated in Fig. 10 (a). When operating temperature increases, the activity driving force across the membrane increases and also the free volume of filled PDMS composite membrane enlargers [33], both are good for diffusion because that makes the polymer chains more flexible (as shown in Fig. 8(b)). Therefore, we can see from Fig. 10 (b), with the increase of temperature, partial fluxes of both water and ethanol increased which resulted in the increase of the total flux. As for the variation trend of separation factor, when temperature was below 60 °C, separation factor increased from 13.1 to 30.1. The reason is that ONS in the membranes made membrane structure changed as analyzed in Section 3.2.1 which makes the absorption and diffusion of ethanol molecules more easier than that of the water molecules (as shown in Fig. 8). But when temperature increased from 60 °C to 70 °C, the slight increase of water solubility selectivity and the sharp decrease of water diffusion selectivity showed in Fig. 8 caused the decrease of the separation factor. Activation energy, Ea, a means to express the effect of temperature on this process, represents the relative change of flux to the change of temperature [34]. When the value of Ea is high, the flux will be more susceptible to the change in temperature. From the experiment results we can see, when the temperature varied from 30 to 70 °C, the total flux increased apparently from 11.1 to 210 g/(m2 · h). According to

D. Sun et al. / Desalination 322 (2013) 159–166

165

Fig. 11. Effects of permeate-side vacuum on pervaporation performance of the 5.0% ONS-PDMS composite membrane: (a) total flux and separation factor; (b) water flux and ethanol flux.

Fig. 10. Effects of operating temperature on pervaporation performance of the 5.0% ONS-PDMS composite membrane: (a) total flux and separation factor; (b)total flux, water flux and ethanol flux;(c) The relation between ln(J) and 1/T.

the solution-diffusion mechanism, Arrhenius type function can be used to express the effect of temperature on flux as follows: J i ¼ J 0 expð−Ea =RT Þ The plots of the total and partial permeation fluxes (ln (Ji)) versus reciprocal temperature (1/T) were shown in Fig. 10 (c) which shows that the variation of the permeation flux to the feed temperature followed the Arrhenius relationship. The activation energy values from the slope are 23.56, 26.70 and 20.42 KJ/mol for total, water and ethanol flux in the 5.0% ONS-PDMS composite membrane which indicated that the permeation of ethanol was more sensitive to the operation temperature than that of water in this 5.0% ONS-PDMS composite membrane. 3.2.3. Effect of permeate-side vacuum on PV performance The effect of permeate-side vacuum on PV performance is another important factor. As showed in Fig. 11, the effect of the permeate-side

vacuum on total flux, ethanol flux, water flux and selectivity of the 5.0% ONS-PDMS composite membrane was examined at feed temperature of 60 °C with a 5 wt.% ethanol–water mixture. When permeate-side vacuum was increased from 0.075 to 0.100 MPa, both total flux and separation factor increased non-linearly as shown in Fig. 11(a). As the permeate-side vacuum increases, the driving force for permeation of both water and ethanol through the membrane increases, which results in a increase of permeation fluxes (Fig. 11(b)). Separation factors rose with the increasing of permeate-side vacuum. That is because, at higher vacuum, the evaporation of ethanol molecules on the surface of permeate–membrane is much easier than that of the water molecules which leads to faster diffusion rate of ethanol molecules than that of water molecules. 4. Conclusions A novel composite membrane using ONS filled PDMS as the top active layer and non-woven fabric as the support layer was developed for the PV of ethanol from water. SEM graphs showed that ONS filled PDMS membranes are dense with no appreciable voids, ONS dispersed uniformly within the PDMS matrix and the top layer of about 80 μm was tightly adhered on the surface of the non-woven fabric substrate. Both XRD and FT-IR observation verified that ONS is only physically blended in the PDMS polymer matrix and the incorporation of ONS into PDMS membrane has a weak effect on the crystalline characterization of filled PDMS membrane. The membrane swelling degree of the ONS-filled PDMS membrane was enhanced with the increase of ONS content in 5 wt.% ethanol aqueous mixtures. With an increase of ONS addition from 0 wt.% to 10 wt.%, the solubility selectivity and the diffusion selectivity have the same trends in which they increased from 21.87 to 38.85 and from 1.87 to 6.18, respectively. When the

166

D. Sun et al. / Desalination 322 (2013) 159–166

temperature ranged from 30 °C to 70 °C, the solubility selectivity increased but the diffusion selectivity first increased then decreased. Incorporation of ONS into PDMS membranes could significantly influence the PV properties of the PDMS membrane for 5 wt.% ethanol aqueous mixtures. With the increasing of ONS content from 0 wt.% to 25 wt.%, because of the interference of ONS particles filled in the PDMS membrane with the tight packing of PDMS chains, total flux increased quickly to the maximum when ONS was 5 wt.% then decreased slowly to the minimum, separation factor increased from 14.1 to 41.2, a three times improvement. As the operating temperature increased from 30 to 70 °C, the permeation flux increased continuously from 11.1 to 210 g/(m2 · h), while the separation factor first increased and then decreased in the 5.0% ONS-PDMS composite membrane using the 5 wt.% ethanol concentration at permeate-side vacuum 0.100 MPa. The variation of permeation flux with feed temperature followed the Arrhenius relationship. When permeate-side vacuum was increased from 0.075 to 0.100 MPa, both total flux and separation factor increased non-linearly due to the increase of driving force and the easier evaporation of ethanol molecules than that of water molecules on the surface of permeate–membrane. Acknowledgments The authors acknowledge the Key Technology R&D Program of China (2006BAE02A01) and Chemistry & Chemical Technology Research Center Plan of Shanghai Huayi Group Company (A200-8608 and A200-80726) for giving financial supports in this project. References [1] N.L. Le, Y. Wang, A.S. Chung, Pebax/POSS mixed matrix membranes for ethanol recovery from aqueous solutions via pervaporation, J. Membr. Sci. 379 (2011) 174–183. [2] S. Chovau, S. Gaykawad, A.J.J. Straathof, B. Van der Bruggen, Influence of fermentation by-products on the purification of ethanol from water using pervaporation, Bioresour. Technol. 102 (2011) 1669–1674. [3] N.R. Gopal, S.V. Satyanarayana, Cost analysis for removal of VOCs from water by pervaporation using NSGA-II, Desalination 274 (2011) 212–219. [4] J. Gu, X. Shi, Y.X. Bai, H.M. Zhang, L. Zhang, H. Huang, Silicalite-filled polyetherblock-amides membranes for recovering ethanol from aqueous solution by pervaporation, Chem. Eng. Technol. 32 (2009) 155–160. [5] X. Jiang, J. Gu, Y. Shen, S.G. Wang, X.Z. Tian, New fluorinated siloxane-imide block copolymer membranes for application in organophilic pervaporation, Desalination 265 (2011) 74–80. [6] S.V. Satyanarayana, A. Sharma, P.K. Bhattacharya, Composite membranes for hydrophobic pervaporation: study with the toluene-water system, Chem. Eng. J. 102 (2004) 171–184. [7] C.H. Cho, K.Y. Oh, S.K. Kim, J.G. Yeo, P. Sharma, Pervaporative seawater desalination using NaA zeolite membrane: mechanisms of high water flux and high salt rejection, J. Membr. Sci. 371 (2011) 226–238. [8] Y. Nagase, T. Ando, C.M. Yun, Syntheses of siloxane-grafted aromatic polymers and the application to pervaporation membrane, React. Funct. Polym. 67 (2007) 1252–1263. [9] C.L. Chang, P.Y. Chang, Performance enhancement of silicone/PVDF composite membranes for pervaporation by reducing cross-linking density of the active silicone layer, Desalination 192 (2006) 241–245. [10] B. Adnadjević, J. Jovanović, S. Gajinov, Effect of different physicochemical properties of hydrophobic zeolites, J. Membr. Sci. 136 (1997) 173–179. [11] X. Zhan, J.D. Li, J.Q. Huang, C.X. Chen, Enhanced pervaporation performance of multi-layer PDMS/PVDF composite membrane for ethanol recovery from aqueous solution, Appl. Biochem. Biotechnol. 160 (2010) 632–642.

[12] M.K. Djebbar, Q.T. Nguyen, R. Clement, Y. Germain, Pervaporation of aqueous ester solutions through hydrophobic poly (ether-block-amide) copolymer membranes, J. Membr. Sci. 146 (1997) 125–133. [13] S. Roualdes, J. Durand, R.W. Field, Comparative performance of various plasma polysiloxane films for the pervaporative recovery of organics from aqueous streams, J. Membr. Sci. 211 (2003) 113–126. [14] A.M. Gronda, S. Buechel1, E.L. Cussler, Mass transfer in corrugated membranes, J. Membr. Sci. 165 (2000) 177–187. [15] Y.I. Park, C.K. Yeom, B.S. Kim, J.K. Suh, J.S. Hong, J.M. Lee, H.J. Joo, Quantitative evaluation of concentration polarization in the permeation of VOCs/water mixtures through PDMS membrane using model equation, Desalination 233 (2008) 303–309. [16] K.H. Yeon, H.H. Won, Influence of ceramic support on pervaporation characteristics of IPA/water mixtures using PDMS/ceramic composite membrane, J. Membr. Sci. 159 (1999) 29–39. [17] Y.W. Huang, P. Zhang, J.W. Fu, Y.B. Zhou, X.B. Huang, X.Z. Tang, Pervaporation of ethanol aqueous solution by polydimethylsiloxane/polyphosphazene nanotube nanocomposite membranes, J. Membr. Sci. 339 (2009) 85–92. [18] X.H. Liu, Y. Sun, X.H. Deng, Studies on the pervaporation membrane of permeation water from methanol/water mixture, J. Membr. Sci. 325 (2008) 192–198. [19] H.L. Zhou, Y. Su, X.R. Chen, S.L. Yi, Y.H. Wan, Modification of silicalite-1 by vinyltrimethoxysilane (VTMS) and preparation of silicalite-1 filled polydimethylsiloxane (PDMS) hybrid pervaporation membranes, Sep. Purif. Technol. 75 (2010) 286–294. [20] Q. Zhao, J.W. Qian, C.X. Zhu, Q.F. An, T.Q. Xu, Q. Zheng, Y.H. Song, A novel method for fabricating polyelectrolyte complex/inorganic nanohybrid membranes with high isopropanol dehydration performance, J. Membr. Sci. 345 (2009) 233–241. [21] B. Li, D. Xu, Z.Y. Jiang, X.F. Zhang, W.P. Liu, X. Dong, Pervaporation performance of PDMS-Ni2+Y zeolite hybrid membranes in the desulfurization of gasoline, J. Membr. Sci. 322 (2008) 293–301. [22] S.H. Chen, R.M. Liou, C.L. Lai, M.Y. Hung, M.H. Tsai, S.L. Huang, Embedded nano-iron polysulfone membrane for dehydration of the ethanol/water mixtures by pervaporation, Desalination 234 (2008) 221–231. [23] D. Yang, J. Li, Z.Y. Jiang, L.Y. Lu, X. Chen, Chitosan/TiO2 nanocomposite pervaporation membranes for ethanol dehydration, Chem. Eng. Sci. 64 (2009) 3130–3137. [24] D. Panek, K. Konieczny, Preparation and applying the membranes with carbon black to pervaporation of toluene from the diluted aqueous solutions, Sep. Purif. Technol. 57 (2007) 507–512. [25] F.B. Peng, Z.Y. Jiang, C.L. Hu, Y.Q. Wang, H.Q. Xu, J.Q. Liu, Removing benzene from aqueous solution using CMS-filled PDMS pervaporation membranes, Sep. Purif. Technol. 48 (2006) 229–234. [26] R.L. Guo, X.C. Ma, C.L. Hu, Z.Y. Jiang, Novel PVA silica nanocomposite membrane for pervaporative dehydration of ethylene glycol aqueous solution, Polymer 48 (2007) 2939–2945. [27] S.J. Lue, C.F. Chien, K.P.O. Mahesh, Pervaporative concentration of ethanol–water mixtures using heterogeneous polydimethylsiloxane (PDMS) mixed matrix membranes, J. Membr. Sci. 384 (2011) 17–26. [28] I.A. Rahman, M. Jafarzadeh, C.S. Sipaut, Synthesis of organo-functionalized nanosilica via a co-condensation modification using g-aminopropyltriethoxysilane (APTES), Ceram. Int. 35 (2009) 1883–1888. [29] U.S. Toti, M.Y. Kariduraganaver, K.S. Soppimath, T.M. Aminabhavi, Sorption, diffusion, and pervaporation separation of water-acetic acid mixtures through the blend membranes of sodium alginate and guar gum-grafted-polyacrylamide, J. Appl. Polym. Sci. 83 (2002) 259–272. [30] Q.L. Liu, J. Xiao, Silicalite-filled poly (siloxane imide) membranes for removal of VOCs from water by pervaporation, J. Membr. Sci. 230 (2004) 121–129. [31] H. Ahn, C.H. Lee, Effects of capillary condensation on adsorption and thermal desorption dynamics of water in zeolite 13X and layered beds, Chem. Eng. Sci. 59 (2004) 2727–2743. [32] B. Adnadjevid, J. Jovanovid, S. Gajinov, Effect of different physicochemical properties of hydrophobic zeolites on the pervaporation properties of PDMS-membranes, J. Membr. Sci. 136 (1997) 173–179. [33] M. Peng, L.M. Vane, S.X. Liu, Recent advances in VOCs removal from water by pervaporation, J. Hazard. Mater. 98 (2003) 69–90. [34] J.G. Wijmans, R.W. Baker, A simple predictive treatment of the permeation process in pervaporation, J. Membr. Sci. 79 (1993) 101–113.